Voice and data transmission cables, such as fiber optic cables and the like are commonly installed within multiple-duct conduit systems which can be laid underground, underwater, or in other locations. Such systems typically are formed in sections laid end-to-end and include an outer duct which houses a plurality of inner ducts in parallel relationship to one another. The duct sections usually are of a defined length, for example, twenty feet long, and are attached in end-to-end relationship. This facilitates installation and maintenance in many cases, such as where the conduit system is installed underground, below roadways or in other confined areas. Typically access to the conduit system is through manholes in the street or ground.
After the multiple duct system has been installed, the fiber optic cables, etc. are installed in the duct system by propelling a carrier attached to an end of a pulling tape through the individual inner ducts, by applying compressed air behind the carrier. When the pulling tape has been drawn by the carrier into one of the inner ducts from one manhole to another, the fiber optic cable is attached to one end of the pulling tape and the cable is drawn by the pulling tape into the inner duct.
When the carriers are propelled through the inner ducts of the conduit system it is important that the inner surfaces of the inner ducts and the joints at the ends of each inner duct of the duct system permit smooth and efficient passage of the carrier so that the carrier can be propelled long distances. This permits the duct system to be constructed with longer distances between its manholes. The smooth and efficient passage of the carrier through the inner ducts is partially dependent on an effective air seal being formed at each joint of the inner ducts and on a relatively smooth side wall of the inner ducts. Also, seals at the ends of the inner and outer ducts protect the duct system from the seepage of water into the duct system so as to avoid contact of moisture with the cables and to avoid the freezing and thawing of water in the ducts.
In some installations, and particularly where the conduit system is to be buried underground and a trench or ditch is dug in the ground to receive the conduit system, rough handling of the conduit system is required to properly install the system. Rough handling of the conduit system is common and occasionally results in loose or faulty connections at the juncture of adjacent conduit sections. Failures at these points can allow water leakage, which can result in corrosion and freezing/thawing problems, and can even result in breakage of one or more cables or wires which is difficult to locate and repair.
In addition, thermal expansion and contraction of the outer and inner ducts can have a detrimental effect on the couplings. For example, when the conduit sections have been assembled, the temperature of the outer duct might change more rapidly than the temperature of the inner ducts, causing expansion or contraction of the outer duct with respect to the inner ducts. Moreover, if the inner ducts are fabricated from a material that is different from the material of the outer ducts, the coefficients of expansion likely will be different for the inner and outer ducts, so that the inner ducts might be relatively longer with respect to the outer ducts at one temperature and relatively shorter with respect to the outer ducts at another temperature. If the multiple duct system is being assembled in hot or cold weather and placed in the ground which could have a temperature difference as much as 30.degree. F. from atmospheric temperature, the temperature of the conduit sections will change after the conduit sections have been placed in the ground and the relative lengths of the inner ducts to the outer ducts will change after the conduit system has been installed. This tends to cause the inner ducts to withdraw from or to protrude further inwardly into the joints formed between the sections of the multiple duct conduit assembly, which can cause an ineffective seal in the joint, or cause damage to the seals of the joints, open joints resulting in leaks in the seals of the joints, cracked joints, etc.
While attempts have been made to solve problems relating to the connections and seals of multiple duct conduit systems, there exists a need in the art for a multiple duct conduit system that includes inner ducts that have inner wall surfaces that offer minimal resistance to the movement of a carrier and to the movement of a fiber optic cable or the like when moved through the ducts, which secures both inner ducts and outer ducts and accommodates movements in the joints of adjacent conduit sections during handling of the conduit sections while still maintaining a good seal at the ends of the conduit sections, a coupling which permits expansion and contraction of the inner ducts with respect to the outer ducts without losing the seals at the joints, which seals the joints of the sections against water, and which is extremely resistent to separation once installed.